Omron CPM1 - PROGRAMING MANUAL 02-2001 Programming Manual page 259

CPM2A/CPM2C Communications Functions
Precautions for Long
Transmissions
(131 characters maximum)
Frame 1
@ 0 0 W
1 0
x10 x10
Node
No.
Frame 2 (128 characters maximum)
One word of data
Data from the same word is not divided.
FCS (Frame Check Sequence)
@
Calculation results
When dividing commands such as WR, WL, WC, or WD that execute write op-
erations, be careful not to divide into separate frames data that is to be written
into a single word. As shown in the illustration below, be sure to divide frames so
that they coincide with the divisions between words.
Data
D
Header
code
Data
When a frame is transmitted, an FCS is placed just before the delimiter or termi-
nator in order to check whether any data error has been generated. The FCS is
8-bit data converted into two ASCII characters. The 8-bit data is the result of an
EXCLUSIVE OR performed on the data from the beginning of the frame until the
end of the text in that frame (i.e., just before the FCS). Calculating the FCS each
time a frame is received and checking the result against the FCS that is included
in the frame makes it possible to check for data errors in the frame.
1 0 R R 0
Node No. Header code
FCS calculation range
Leftmost
ASCII code
@ 40 0100
1 31 0011
0 30 0011
R 52 0101
0 30 0011
0 30 0011
to
0 30 0011
1 31 0011
0100
4
One word of data
Data from the same word is not divided.
FCS
0 0 1
Text
Rightmost
0000
XOR
0001
XOR
0000
XOR
0010
XOR
0000
XOR
0000
XOR
0000
XOR
0001
0010
Converted to hexadecimal.
Handled as ASCII characters.
2
Section
FCS
*
Terminator
4 2
*
FCS Terminator
4-3
Delimiter
239